You can transfer your mails from the old AU mail system to the AU Google Apps, which I will call it Gmail in this document.
The concept of getting your old mails is to download them via POP3 service. This is very similar to setting your mail client service.
In this case, the Gmail itself is a mail client and the mail server is old AU mail system.
However, the POP3 can download only the mails stored in Inbox folder, if you have some custom folders, please follow this link first.
To create a POP3 account, please follow the steps shown below.
Step 1: Login to home.au.edu and open Gmail.
Step 2: At Gmail page, click at the "Setting" icon (see the picture) and then select "Settings".
Step 3: Select tab “Account” and then at the “Check mail from other accounts (using POP3)” select “Add POP3 mail account you own”
Step 4: A POP3 account adding dialog will be displayed as shown below. At the first wizard page, enter your username with “@backup.au.edu” as email account and then click Next Step.
Step 5: Enter your username and password (existing one).
Step 6: Enter “pop3.au.edu” as POP3 Server
Step 7: Click to add check mark at “Leave a copy …” check box.
Step 8: Click "Add Account"
Step 9: Then the system will ask you to use the email@backup.au.edu as a sender, please just say "No" and click "Finish".
Now, the Gmail will attempt to fetch mails from old AU system (pop3.au.edu) anytime you logged in to the Gmail.
The fetching process may stop after you logged out, but will resume again when you logged in.
It may cannot download every mail at once, so please give it some times to download.
NOTE
Some of your mails may be considered as SPAM, so please check at "Spam" box to see if your old mails are there.